Many high energy technologies such as fusion energy and lasers, generate large quantities of waste heat within short time periods which must be removed quickly to prevent damage to the equipment. Other devices may require quick removal of thermal energy and may later require an input of thermal energy to maintain temperature. This temperature management can be achieved by the use of a thermal energy storage unit.
In general, thermal energy storage units or thermal capacitors must be able to rapidly store large quantities of heat at low cost and then return the energy source on to an energy dump for dissipation. For space applications, system size is also an important consideration in determining cost and overall system efficiency. A most efficient way to store thermal energy rapidly is by using the "heat of fusion" phenomenon. More specifically, as a material melts, it absorbs relatively large quantities of energy, thereby cooling the heat exchange medium. In the reverse process, the phase change material cools down and freezes, or solidifies, thereby releasing large quantities of energy. Phase change substances commonly used for this purpose include crystalline polyolefins, chlorides and nitrates as well as other materials. For applications at higher temperatures, metals can be used, particularly sodium, lithium, and various alloys of bismuth, lead, tin, cadmium and indium.
Thermal energy storage units using a phase change material usually consist of a vessel filled with bulk phase change material with a a metal pipe heat exchanger embedded therein. A fluid such as water is passed through the heat exchanger to transfer heat into and out of the phase change material. The heat transferred is limited by the surface of the metal pipe heat exchanger.
In recognition of the above described limitation, thermal energy storage units were developed wherein the heat absorbing substance is encapsulated to increase the heat transfer surface area. However, these systems are not without their drawbacks as the pressure required to pump the heat transfer medium through the apparatus is relatively high. The higher pumping pressure requires an increase in energy consumption by the pump. When pumping pressure is high, capsule breakage becomes a problem with systems of this type. It is important that the encapsulating shell material remain solid and structurally sound throughout the heat exchange process to prevent the melted material contained in the capsule from coalescing into larger masses and flowing out of the thermal energy storage unit, or plugging orifices. In addition, should the capsules degrade or rupture, system contamination and degredation of system performance will occur.
In view of the foregoing it is an object of the present invention to provide a thermal energy storage apparatus which has a low pressure drop.
Another object of the invention is to provide a thermal energy storage apparatus with low energy requirements to pump the heat exchange medium.
Another object of the present invention is to provide a thermal energy storage device having a high capacity for thermal energy absorption and/or release.
A further object of the invention is to provide a thermal energy storage apparatus wherein the capsules containing the heat storage material are less likely to break, thereby decreasing the risk of contamination and increasing system stability and reliability.